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Abstract

In order to realize the potential circuit advantages (such as logic compression) offered by quantum well devices, it is important to successfully achieve both electron and hole tunneling in Si/SiGe. We disclose different quantum well structures using strain engineering to achieve electron tunneling in Si/SiGe.

Country

United States

Language

English (United States)

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This is the abbreviated version, containing approximately
52% of the total text.

In order to
realize the potential circuit advantages (such as
logic compression) offered by quantum well devices, it is important
to successfully achieve both electron and hole tunneling in Si/SiGe.
We disclose different quantum well structures using strain
engineering to achieve electron tunneling in Si/SiGe.

INTRODUCTION

Hole
tunneling structures in the Si/Si1-xGex material system
have been proposed and fabricated [1-3].
This is intuitively a
simpler structure to envision since the difference in band gap
between Si and SixGe1-x manifests itself primarily in the valence
band edge.

In order to
exploit the advantages of electron transport (high
mobility and low effective mass) and permit realization of
complementary logic quantum mechanical devices, it is important to
achieve both electron and hole tunneling.
INVENTION

We show that
by engineering the relative strain in Si/SiGe
heterostructures, it is possible to use the strain-induced change in
band lineup to realize electron tunneling structures in the Si/SiGe
system. Fig. 1a shows the conduction
band edge of a (100) fully
relaxed Si0.75Ge0.25 quantum well with strained Ge barriers. The
six-fold W valleys in the Ge are split due to compressive strain into
four-fold and two-fold states with the four-fold states moving down
from their degenerate position and the two-fold states moving up
[4,5]. This causes the effective conduction
band offset between
lowest lying states in the Si0.75Ge0.25 and Ge to be considerably
reduced (around 0.1 eV). Further, the
effective mass perpendicular
to the heterointerfaces (in the direction of transport) is much
larger than parallel to the interfaces.
Consequently, there is a
large current associated with transport through the low effective
mass, four-fold states in the Ge barriers which then results in poor
peak-to-valley current ratios in this structure. This is despite the
fact that the perpendicular conduction band offset between the
Si0.75Ge0.25 and the two-fold Ge states is quite large (around 0.5
eV). TEM cross-sections of a
Si0.75Ge0.25 quantum well with Ge
barriers reveal the interfaces in this structure to be abrupt (within
TEM resolution), but the I-V characteristics obtained from this
device are poor.

In the (111)
direction, the conduction band offset between W
valleys in Si0.75Ge0.25 and Ge is around 0.25 eV. However, the eight-
fold L valleys in the Ge are split due to strain, and the lowest
lying conduction band in the Ge barrier are the six-fold L minima
with a conduction band offset between Si0.75Ge0.25 and Ge of around
0.15 eV (see Fig. 1b). In this case
direct tunneling of electrons
through the higher...